Separation method



SEPARATION METHOD Filed Feb. 27, 1968 wtmll mw /NvE/vrons SABR/ E RGU N MART /N HERMAN s lulllllll'lll'dll HHH HHHIHIIH HHUII'IH UVHHIIIIIHUH HIIH HIIIIH H H... |||||I|IY kum United States Patent O 3,463,310 SEPARATION METHOD Sabri Ergun and Martin Berman, Pittsburgh, Pa., assignors to the United States of America as represented by the Secretary of the Interior Filed Feb. 27, 1968, Ser. No. 708,627 Int. Cl. B03c 1/16, 1/00 U.S. Cl. 209-8 14 Claims ABSTRACT OF THE DISCLOSURE This invention resulted from work done by the Bureau of Mines of the Department of the Interior, and the domestic title to the invention is in the government.

BACKGROUND OF INVENTION It is known that the magnetic properties of many minerals can be altered by subjecting them to a heat treatment. This is the simplest and probably the only practical method for altering magnetic properties. The process may be a simple heating, an oxidizing roast as for many sultides, a reducing roast as for hematite and other iron oxides or a combination of several treatments. After such heat treatment, the mineral being procesed is conventionally subjected to a magnetic separation step.

Some of the minerals which are known to display increased magnetic activity after heat treatment include pyrite, hematite, marcasite, siderite, chalcopyrite, arsenopyrite, bornite, pyrolusite and many others. Temperatures used in the heat treatment generally range from about 300 to 1000 C. for a contact time of few seconds to an hour or more. Treatment conditions used include oxidizing, inert and reducing atmospheres depending upon the mineral. Heat treatment followed by magnetic separation now finds its greatest use in ore concentration.

It has been proposed to use this same process for the removal of pyrite from coal. Sulfur content of bituminous coals mined in the United States ranges from less than 1% to as much as 6% or more. Pyritic sulfur generally makes up from about 40 to 80% of the total sulfur present. Recent emphasis on the reduction of sulfur dioxide emissions from coal burning processes is forcing utilities and other coal users to rely either on scarce, naturally occurring low sulfur coals or on coals which have been processed to lower their sulfur content.

Finely divided coal can be subjected t a thermal treatment, usually in the presence of steam and air, to effect at least a partial conversion of the contained pyrite particles to pyrrhotite, magnetite and gamma-hematite. Because these minerals are all ferromagnetic while the original pyrite is paramagnetic, the treatment allows an elficient magnetic separation of the altered pyrite from the coal.

The pyrite-pyrrhotite transition takes place very rapidly at temperatures on the order of 600 C. and the pyrite to magnetite or hematite oxidation reaction also requires relatively high temperatures. Coal treatment temperatures as high as 360 C. have been used and alteration of the surfaces of pyrite grains to magnetic forms has been reported at these conditions.

In spite of the apparent attractiveness of this process for the removal of pyrite from coal, it has two major 3,463,310 Patented Aug. 26, 1969 ICC drawbacks which seriously limit its applicability. First, the cost of heating the coal makes the process economically unattractive. Secondly, and most important, heating coal above about 200 C. starts to drive ott the coal volatiles. Besides changing the burning characteristics 0f coal, volatiles lost from the coal represent a significant portion of its calorific value.

The process of this invention takes advantage of the conductivity differences to electromagnetic radiation between pyrite and coal in order to selectively heat the surface of pyrite particles contained within the coal. C011- version of at least the surface of pyrite particles to ferromagnetic forms is accomplished without substantial heating of the coal.

It is an object of this invention to selectively alter the properties of one member in a physical mixture by subjecting the mixture to electromagnetic radiation.

Another object of this invention is to alter at least the surface of a diamagnetic or paramagnetic mineral to ferromagnetic forms by selectively heating that mineral while it is in physical admixture with other components.

A specific object of this invention is to remove pyrite from coal.

DESCRIPTION OF THE INVENTION The invention will be more clearly understood from the following description of a preferred embodiment wherein reference is made to the accompanying drawing.

The figure is a schematic flow diagram of a preferred embodiment of the separation process.

Referring now to the figure, there is shown an arrangement particularly adapted for the treatment of coal to remove the contained pyrite. Finely-divided coal from feeding hopper 11 is deposited upon moving conveyor belt 12 by means of feeding device 13 and distributor 14. The particle size range of coal fed t0 the process should conform as nearly as is conveniently possible to the size range producing maximum pyrite availability, or substantial physical release of pyrite crystals from the coal matrix. Since the size of pyrite particles is known to vary widely between different types of coals, the preferred size range is dependent between different types of coals, the preferred size range is dependent primarily upon the particular coal being processed. Particle size range producing maximum pyrite availability will vary generally in the broad range of about 40 to 400 mesh. `In Pittsburg seam coal for example, maximum pyrite availability occurs in the range of 200 to 400 mesh.

Tunnel irratiator 15 is of rectangular cross-section with electrically conducting walls. End sections 16 and 17 terminate the tunnel section and function to suppress the escape of electromagnetic energy. Coal is carried through tunnel 15 by continuous belt conveyor 12 formed of a dielectric material such as a reinforced Teflon. Conveyor 12 may be mounted on rotating drums 18 and 19 disposed one at each end of the tunnel as is conventional in the art. A driving force is applied to at least one of the drums in the direction indicated by arrow 20.

Electromagnetic energy, produced by a suitable conventional source 21, is introduced into tunnel 15 by means of energy conduit 22. At microwave frequencies, conduit 22 comprises a waveguide of rectangular cross section having a series of openings spaced along the wall facing the interior of tunnel 15. Microwave energy is thus directed downwardly into the interior of the tunnel where it interacts with the pyrite and coal. Depending upon the frequency of electromagnetic radiation utilized, coaxial cables, striplines and the like may be utilized as the energy conduit rather than the waveguide.

Treatment time of the coal within the irradiating cavity or tunnel 15 is dependent upon the moisture content of the coal, the frequency of the electromagnetic radiation and the effective voltage gradient within the tunnel. Treatment times in the range of 1 to about 100 seconds are satisfactory for most coals over the frequency range investigated. At 10 ghz, exposure times of about 2 to 10 seconds were found satisfactory.

While preferential heating of pyrite dispersed in coal occurs over a wide range of the electromagnetic spectrum, it is preferred to operate at frequencies above 106 cycles per second or 1 mhz. In a most preferred embodiment, frequencies used are above 109 cycles per second or 1 ghz.

Optimum process efliicenies are obtained when only surface modilication of the individual pyrite particels to ferromagnetic forms has been achieved. Continued exposure results in more complete reaction of the pyrite but also results in undue heating of the coal. It is preferred that exposure time be limited to a pyrite conversion level of less than 10% and most preferably to a pyrite conversion level of less than 5%.

In some instance, it is desirable to provide a controlled' ow of treating gas through the tunnel 1S. Gas may be introduced via conduit 23 at one end of tunnel 15 and removed via conduit 24. In processing coal for the removal of pyrite, the treating gas preferably is air. Use of air as the treating gas performs two functions; it sweeps out water vapor released from the coal and it provides a EXAMPLE Samples of finely divided coal containing pyrite were subjected to electromagnetic radiation at frequencies ranging from 400 khz to ghz. Throughout this frequency range, preferential heating of the contained pyrite was observed. As the frequency increased, the preferential heating of pyrite relative to coal become more pronounced. Pyrite in the mixture thus treated was strongly attracted to a magnet and no losses of coal volatiles were observed.

Good conversion of the pyrite particles to ferromagnetic forms was observed at 10 ghz without noticable heating of the coal. The trend of the data indicated that even higher frequencies would increase the selectivity of the heating process.

While the process has been illustrated using a conveyor-tunnel type of apparatus, the electromagnetic treatment may be performed using any other applicable type of high frequency heating equipment. For example, a coaxial tube arrangement having an annular treatment chamber with the electromagnetic radiation facilities arranged within the inner tube may also be used to advantage The process has been illustrated as being particularly applicable to the removal of pyrite from coal. By appropriate choice of radiation frequency, separations may be performed on other mineral mixtures. Generally, any mineral mixture having at least one member which is susceptible to being altered to a more highly magnetic form upon heating can be treated by this process. One example of such a mixture is a pyrite-chalcopyrite-silicate ore which may be treated to produce a copper concentrate.

What is claimed is:

1. A process for the separation of a mixture of particulate materials, at least one of said materials being susceptible to a change in its magnetic properties upon heating, which comprises subjecting said mixture to electromagnetic radiation having a frequency whereat said material susceptible to a change in magnetic properties absorbs electromagnetic energy `more strongly than does the other components in said mixture and is thereby preferentially heated, continuing the irradiation for a time sufficient t0 cause conversion of at least a surface layer of said particulate material being susceptible to such a change to a more highly magnetic form, terminating the irradiation before substantial heating of the other components of the mixture occurs, subjecting said irradiated mixture to a magnetic separation step and recovering a portion enriched in said magnetically altered material.

2. The process of claim 1 wherein the conversion of said magnetically alterable material is less than 10%.

3. The process of claim 1 wherein said frequency is greater than 1 mHz.

4. The process of claim 1 wherein said mixture is contacted with a How of treating gas during said irridation step.

5. The process of claim 1 wherein said mixture comprises coal and said material susceptible to a change in magnetic properties upon heating is pyrite.

6. The process of claim 5 wherein said more highly magnetic form comprises pyrrhotite.

7. The process of claim 5 wherein said frequency is greater than 1 mHz.

8. The process of claim 7 wherein said frequency is greater than 1 gHz.

9. The process of claim 5 wherein said mixture is contacted with a ow of treating gas during said irridation step. y

10. The process of claim 9 wherein said treating gas comprises air.

11. The process of claim 5 wherein the conversion of said pyrite is less than 10%.

l2. The process of claim 11 wherein the conversion of said pyrite is less than 5%.

13. The process of claim 5 wherein the particle size of said coal is less than about 40 mesh.

14. The process of claim 13 wherein the particle size range of said coal is less than about 200 mesh.

References Cited UNITED STATES PATENTS 1,478,295 12/1923 Perkins 209-11 1,512,870 10/1924 Ullrich 209-214 2,907,456 10/ 1959 Brison 209-11 3,097,160 7/1963 Rich 20'9-11 X FOREIGN PATENTS 67,456 5/ 1927 Sweden.

OTHER REFERENCES Aronov et al., Coke Production, Heating Coals with High Frequency Currents, Stal. 15, 771-776, 1955.

Kester, Magnetic Demineralization of Pulverized Coal, Mining Eng., May 1965, pp. 72-76.

HARRY B. THORNTON, Primary Examiner R. HALPER, Assistant Examiner U.S. Cl. X.R. 209-214, 11 

